Support full netcdf4 features

[pp-mo]

That is,, structural features anyway (i.e. not including compression, chunking + the like).
Which in full is ...

• groups
• extended basic types
• multiple unlimited dimensions
• user-defined types
  • variable-length types (see separate Support variable-length types #94 )
  • enum types
  • compound types (i.e. like data records, C structures, or numpy recarrays)
  • opaque types

---

NOTE:

Some of this is not supported by the python netCDF4 module.

• XX opaque types
• XX usertypes WITHIN usertypes,
  • e.g. compounds containing vlens or enums (annoyingly, means no strings or enums in record-like structures)
  • e.g. variable-length content of compounds (structures)
• XX can't preserve file-ordering of usertypes, as python layer doesn't provide access to this
  • (though might be possible via private properties ??)

See here ...

Compound (struct), variable length (vlen) and enumerated (enum) data types are supported, but not the opaque data type.
Mixtures of compound, vlen and enum data types ... are not supported.